氨氮胁迫下凡纳滨对虾对副溶血弧菌的易感性

摘要

Acute Hepatopancreatic Necrosis Syndrome (AHPNS), also know as Early Mortality Syndrome (EMS), was officially reported in China in 2010 and has caused large-scale die-offs of cultivated shrimp in several Asian countries. A research team of the University of Arizona has identified that the pathogen is Vibrio parahaemolyticus which is a strain of a bacterium commonly found in brackish coastal waters around the globe. Because V. parahaemolyticus is a conditioned pathogen, it is very important to study the effects of ammonia nitrogen on the toxicity of this pathogen to Litopenaeus vannamei. To evaluate the effects of total ammonia nitrogen during the outbreak of AHPNS in L. vannamei, we examined the susceptibility to the pathogen, the non-specific immunity and the expression of LvLT mRNA in the hepatopancreas of the shrimp. Shrimp underwent the ammonia stress for 20 days with different ammonia nitrogen concentrations:2.5 mg/L, 5.0 mg/L, 7.5 mg/L, 10.0 mg/L and the control concentration. After the exposure to V. parahaemolyticus, the death rate of shrimp perked in 6–24 h. The accumulative mortality rates of the treated groups were 0, 8%, 12%, 20%and 36%respectively at 120 h. The activities of phenoloxidase (PO) reached the lowest at 12 h in the control, 2.5 and 5.0 mg/L groups, and at 24 h in the 7.5 and 10.0 mg/L groups. The activity of superoxide dismutase (SOD) increased at first and then gradually declined, and lysozyme (LSZ) exhibited the same trend. The expression of LvLT mRNA in the hepatopancreas increased at 6 h in all groups and was significantly higher in the control, 2.5, 5.0 and 7.5 mg/L group than that in the 10.0 mg/L group. The LvLT mRNA expression reached the maximum at 12 h and then declined gradually at 24 h. The results indicated that high ammonia nitrogen could cause depression in the immunity of L. vannamei, and subsequently increase their susceptibility to V. parahaemolyticu and the resultant mortality rate. Therefore it is crucial to regulate the concentration of total ammonia nitrogen in aquatic environment to prevent the breakout of Acute Hepatopancreas Necrosis Syndrome.%为探讨养殖水体中总氨氮胁迫对凡纳滨对虾(Litopenaeus vannamei)“急性肝胰腺坏死综合征(Acute Hepatopancreatic Necrosis Syndrome, AHPNS)”发生的影响，设置了1个对照组和4个不同质量浓度氨氮实验组：2.5、5.0、7.5、10.0 mg/L实验组，胁迫20 d后腹肌注射副溶血弧菌(Vibrio parahaemolyticus)进行感染。凡纳滨对虾感染后6–24 h，除对照组外，各实验组均出现死亡高峰。24 h后，5.0、7.5和10 mg/L实验组对虾累积死亡率均高于2.5 mg/L组，且在同一取样时间各实验组对虾累积死亡率随着氨氮浓度的升高而升高，48 h后各实验组对虾不再死亡，其累计死亡率分别为0、8%、12%、20%和36%。PO活性呈现先升高再降低的趋势，对照组、2.5和5.0 mg/L实验组PO活性差异性不显著(P>0.05)，7.5和10 mg/L实验组除12 h外均显著低于对照组(P<0.05)。SOD活性呈现先升高后下降的趋势，7.5和10 mg/L实验组SOD活性在感染后除24 h外均显著高于对照组(P<0.05)，而2.5 mg/L实验组与对照组差异性不显著(P>0.05)。对照组和2.5 mg/L实验组对虾LSZ活性在各取样时间点差异性不显著(P>0.05)，7.5和10 mg/L实验组对虾LSZ活性在3 h、6 h、24 h、48 h时间点显著低于对照组(P<0.05)。感染6 h后各实验组对虾肝胰腺中LvLT mRNA表达量开始上升，24 h后开始下调，至72 h恢复至原水平。实验结果表明，氨氮胁迫能降低凡纳滨对虾的非特异性免疫酶活性，影响对虾肝胰腺中LvLT mRNA对病原刺激的应答反应，增加对副溶血弧菌的易感性，为预防凡纳滨对虾AHPNS的暴发，养殖水体中总氨氮浓度应控制在1.96 mg/L以下。